1. Field of the Invention
The present invention relates to a single-mode optical fibre and in particular to a single mode optical fibre exhibiting low bending losses.
2. Description of the Related Art
The use of single-mode optical fibres in fibre-to-the-premises (FTTP) applications, including fibre-to-the-home (FTTH) and fibre-to-the-building (FTTB) applications, generally require low bending loss of optical signals transmitted through the fibres, also under stringent installation constraints that may impose tight bend radii, e.g., due to sharp cornering in buildings or compression of optical fibres. In particular, cabling and hardware applications aimed to miniaturize passive field equipment, e.g., local convergence cabinets or storage boxes, and the development of multi-dwelling units (MDUs) require fibre designs with superior bending capabilities. In addition, coarse wavelength division multiplexing systems (CWDM) and passive optical network (PON) systems may also need employment of bend-insensitive optical fibres.
Recently, microstructured optical fibres have been developed for single-mode transmission and low bending loss. These fibres typically include a solid central core surrounded by a hole-containing silica cladding, wherein the holes are arranged in a random or non-periodic spatial distribution.
WO 2008/005233 discloses a microstructured optical fibre comprising a core region and a cladding region surrounding the core region, the cladding region comprising an annular hole-containing region comprised of non-periodically disposed holes. The core region and cladding region is said to provide improved bend resistance and single mode operation at wavelengths greater than or equal to 1500 nm, in some embodiments greater than 1260 nm. The fibres disclosed in this document are said to be possibly produced by relatively low cost manufacturing process, because expensive dopants such as fluorine and/or germania can be avoided in the hole-containing region, if desired, and the stack and draw manufacturing process of arranging spatially periodically disposed holes in the glass part of the optical fibre can likewise be avoided, if desired. It is further mentioned that, alternatively, the methods disclosed herein can be used simply to add holes or voids to a cladding of a fibre which is doped with one or more of germania, phosphorous, aluminium, ytterbium, erbium, fluorine or other conventional fibre dopant materials, or which also contains spatially periodically disposed holes in the cladding, to increase the bend resistance thereof.
A microstructured optical fibre in which the cladding region comprises an annular void-containing region comprised of non-periodically disposed voids is described also in WO 2009/099579.
Fluorine-doped optical fibres can be tailored to a number of different applications. U.S. 2009/0185780 relates to a transmission fibre with an improved resistance to high-dose radiation having a refractive index that is uniformly depressed with respect to the profile of a standard single-mode fibre.
U.S. 2003/0200770 discloses a method of making a fluorine-doped soot. The ability to include fluorine in a preform is generally considered an important aspect of producing an optical fibre with a fluorine-doped region. Deposited fluorine has shown to be a volatile compound and to exhibit a significant migration from the regions of interest. The solution described in this document is said that it can be used to increase the concentration of fluorine doping species in an atmosphere for fluorine doping a soot particle during deposition.
U.S. Pat. No. 7,555,187 relates to an optical fibre having an effective area larger than 95 μm2 and bend loss of ≦0.7 dB/turn on a 20 mm diameter mandrel. The disclosed fibre comprises a glass core and a glass cladding comprising a first and second annular region and a third annular region (outer region), wherein the second annular region comprises a minimum relative refractive index relative to the third annular region lower than zero, preferably equal to or lower than −0.3%. The document states that, in a set of embodiments, the second annular region comprises silica based glass (either pure silica, or silica doped with for example, germanium, aluminium, phosphorus, titanium, boron, and fluorine) with a plurality of closed randomly dispersed holes, which provide an effective refractive index which is low, e.g. compared to pure silica.
Large effective area optical fibres have been used for long-distance telecommunication systems, as generally a large effective area reduces the non-linear optical effects. However, an increase in effective area is known to typically result in an increase of macrobending induced losses.
U.S. 2008/0279515 relates an optical fibre comprising a silica-based core and cladding, wherein the core comprises an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof in an average concentration in said core between about 10 and 10000 ppm by weight. The cladding which surrounds the core includes at least a first annular region having an index delta percent lower than that of the core, and a second annular region having an index delta percent lower than that of the first annular region. The second annular region is said to preferably comprise randomly distributed voids, fluorine, or mixtures thereof.
An optical fibre including a void-containing region with a random void distribution can be manufactured during formation of the preform by a sintering process in which gases with low-solubility in the materials forming the fibre, usually silica-based materials, remain trapped and form voids. The preform can be manufactured in two main steps: first, a glass core rod including the preform core, which is preferably void-free, is produced by deposition and then consolidated, and, second, a preform outer cladding is formed around the glass core rod by deposition and then consolidated to form voids within the preform cladding layer. The resulting consolidated preform typically exhibits an annular void-containing region including a random distribution of voids, which starts at about the outer surface of the core glass rod and extends radially within the cladding for a certain radial thickness. Radial thickness (or width) of the void-containing annular region and local void density within the annular region may widely vary in dependence on the sintering process conditions, such as consolidation time, temperature gradient in the furnace and percentage of volume of low-solubility gases during consolidation.
The drawing process following the formation of the preform, in which the preform glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fibre, have an effect on the voids. Typically, the stretching of the preform into the optical fibre changes the void shape from spherical to elongated.
In a microstructured optical fibre with a random-void distribution, bend resistance can depend on the thickness of the void-containing region and/or on the density of the voids in said region. In particular, bend resistance has been seen to be correlated with the product of the local void density and the cross-sectional area of the void-containing region. Generally, the higher the density of the voids, the larger the depth of the refractive index with respect to the outer cladding region, taken as a reference refractive index value, in other words, the larger in absolute value the minimum relative refractive index of the depressed-index void-containing region.
Bend resistant single-mode optical fibres with an annular ring region of low relative refractive index are disclosed in U.S. Pat. No. 7,450,807. In one set of embodiments, the annular ring region comprises silica glass having a dopant selected from the group consisting of germanium, aluminium, phosphorus, titanium, boron, and fluorine. In another set of embodiments, the annular ring region comprises silica glass with a plurality of holes.
The profile volume, V3, of the annular ring region extending outwardly from an inner radius R2 to an annular ring region radius R3 is defined in U.S. Pat. No. 7,450,807 as:V3=2∫R3R2Δ3(r)dr  (1)where Δ3 is the relative refractive index across the region.